Technical Field
The present invention relates to an optical measuring apparatus and method and particularly relates to an absolute distance measurement apparatus and method using laser interferometric wavelength leverage.
Description of Related Art
With the development of science and technology, absolute distance measurement of large length with high accuracy are applied more and more widely to high-end equipment manufacturing, spatial engineering, meteorological technique and so on. For example, the measurement of racks of heavy machines, the measurement of beds of large precision machine tools, the measurement of lengths of steam and hydraulic turbine spindles, the measurement of diameters of stators and rotors of water-turbine generator sets of hydropower stations, the measurement of installing positions of aircraft fixtures in the aerospace industry, the monitoring of positions and postures and the high-resolution distance measurement of satellites in satellite formation flying and so on not only demand distance measurement accuracy to reach micrometer level and even lower in the scope from few tens of meters to hundreds of meters but also need these measurement instruments to have high efficiency and good flexibility and to be suitable for measuring conditions without guide rails.
The absolute distance measurement methods are mainly divided into two types: time-of-flight measurement and interferometric measurement. Among the time-of-flight measurement, the distance measurement with laser pulses is limited by picosecond time measurement accuracy, and thus the distance measurement accuracy is in the millimeter level; although the measurement accuracy of the balanced optical cross-correlation based on femtosecond pulses superposition can realize the sub-micrometer order, the measured distance must be an integer multiple of the femtosecond interpulse distance, then arbitrary absolute distance cannot be measured; the phase shifting method achieves the time of flight by measuring the phase delay generated by the measured round-tripping distance of modulated light waves, the measurement resolution depends on the phase resolution of the maximum modulating frequency, and the measured distance is limited by the non-ambiguity distance corresponding to the minimum modulating frequency. The laser interferometric absolute distance measurement mainly includes frequency-sweeping interferometry, femtosecond pulse dispersive interferometry and multiple-wavelength interferometry. For frequency-sweeping interferometry, the variations of the measured distance will induce large errors during the sweep, and the relative uncertainty is 10−6; the femtosecond pulse dispersive interferometry is limited by the resolution of spectrum analyzer, and the relative uncertainty is 10−5; the multiple-wavelength interferometry uses multiple wavelengths to constitute a gradually increasing synthetic-wavelength chain and gradually obtains the measured distance by starting from the maximum synthetic wavelength according to the initial estimate of the measured distance and the fractional fringe order corresponding to each synthetic wavelength. In the multiple-wavelength interferometry, the fractional fringe order is usually determined by using beat-wave detection, heterodyne detection, superheterodyne detection and son on, wherein the beat-wave detection is affected by direct-current drift of light intensity, and relative uncertainty is less than 10−6; in the optical configurations of the heterodyne detection and the superheterodyne detection, heterodyne light sources are affected by frequency modulators and the stability of synthetic wavelength is low, and thus the distance measurement accuracy is difficult to improve.